U.S. stocks switched between gains and losses on Wednesday as investors remained skittish ahead of an announcement from President Donald Trump's administration's on taxes.

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Early gains, however, were still enough to help the Nasdaq Composite Index to touch an intraday all-time high at 6,039.28, a day after the tech-laden index surpassed the psychologically important level of 6,000 for the first time ever.

The S&P 500 index briefly traded above its previous closing high set March 1, but pared back to trade up 4 points, or 0.2%, at 2,392. Health-care, telecom and energy shares were leading gains, while the consumer-staple and real-estate sectors were the only ones trading lower. Energy shares rebounded, following higher oil prices.

The anxious trading atmosphere comes after stocks on Tuesday climbed sharply higher (http://www.marketwatch.com/story/us-stocks-poised-to-build-on-rally-with-flood-of-earnings-ahead-2017-04-25), on the back of upbeat earnings and the possibility of significant corporate tax cuts (http://www.marketwatch.com/story/trumps-15-corporate-tax-rate-could-cost-the-government-2-trillion-2017-04-25).

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"Nobody is expecting anything other than a broad outline of the tax reform, but that will have more clarity than the previously vague description like 'massive' or 'phenomenal'," said Quincy Krosby, market strategist, at Prudential Financial.

Krosby noted that earnings, especially revenue growth has been underpinning the market as well as optimism about tax reform.

Trump won't present the tax announcement himself, but will leave it to Treasury Steven Mnuchin and National Economic Director Gary Cohn, according to media reports (http://thehill.com/policy/finance/330588-mnuchin-cohn-expected-to-roll-out-trump-tax-plan). The press briefing at the White House is scheduled for 1:30 p.m. Eastern Time.

Mnuchin, on CNBC before the market open confirmed that the White House tax plan will include a 15% corporate tax rate, describing the planned overhaul as the biggest tax cut in history.

Read:Trump's tax reform could lift the stock-market to new heights (http://www.marketwatch.com/story/trumps-tax-reform-could-lift-the-stock-market-to-new-heights-2017-04-26)

Promises of a tax plan have been one of the linchpins of recent gains for risk assets and is likely to be a significant driver of trading action across asset classes when the blueprint of the plan is released later.

"We will be closely watching the dollar and Treasury yields after the tax plan unveiled," Krosby said.

The dollar traded mostly higher against other major currencies ahead of the announcement on Wednesday, with the ICE Dollar Index up 0.4% at 99.177. The yield on the 10-year Treasury note fell 1 basis point to 2.32%.

Some are anticipated a more muted reaction to any tax announcement, citing Trump's recent struggles getting a bill to rework Obamacare to a House vote.

"Given the failure in the first 100 days to get anything accomplished legislatively, I doubt Wall Street is going to go out and party on the assumption that Republicans will be able to translate today's wish list into tomorrow's legislation," wrote James Meyer, chief investment officer at Tower Bridge Advisors in a note.

Also in focus was a potential government shutdown this weekend. Some of those concerns, however, may have eased after Trump backed off on a demand for funding for a wall between the U.S.-Mexico border (http://www.marketwatch.com/story/trump-backs-off-demand-for-immediate-border-wall-funding-2017-04-24).

See:Should Wall Street fear a government shutdown? Here's how stocks fared in the past (http://www.marketwatch.com/story/should-wall-street-fear-a-government-shutdown-heres-how-stocks-fared-during-past-closures-2017-04-21)

Earnings season: The season continues in full swing, with a deluge of results. Shares of PepsiCo Inc.(PEP)fell 1.8% even as earnings beat analyst forecast.

After the market closes, earnings from Amgen Inc.(AMGN), Tractor Supply Co.(TSCO) and PayPal Holdings Inc.(PYPL) are on tap.

Other markets:Stocks in Asia closed mainly higher (http://www.marketwatch.com/story/asia-pacific-markets-continue-to-ride-wave-of-gains-2017-04-25), propelled by optimism over the prospect of a U.S. tax overhaul.

European markets were slightly lower (http://www.marketwatch.com/story/european-stocks-struggle-to-hold-onto-20-month-high-ahead-of-trump-tax-unveiling-2017-04-26), after surging to historic highs following the first round of the French presidential election on Sunday. Centrist Emmanuel Macron came out on top and will battle against far-right Marine Le Pen in the second round on May 7--a vote Macron is expected to win.

Read:Emmanuel Macron: 5 things to know about the man poised to be France's president (http://www.marketwatch.com/story/emmanuel-macron-5-things-to-know-about-the-man-poised-to-be-frances-president-2017-04-24)

Oil prices (http://www.marketwatch.com/story/oil-prices-slip-again-as-us-supply-worries-resurface-2017-04-26-11033520) were lower ahead of the U.S. Energy Information Administration's supply data out later on Wednesday.